first, draw the complete line on a layer above the JPG.
then on a layer just above the line, draw a rectangle that
covers it completely. convert it too a symbol (F8.) Right clicl on
the layer and set this layer to Mask.
Add enough frames to all layers for your desired animation
On the Mask layer, add a key frame at the last frame.
On the Mask layer, on the first frame, scale the mask
rectangle Horizontally so that it only masks the first part of your
Right click on the Mask layer and Create Motion tween. you
may have to adjust the Center poitn of your recangle mask (the
small white dot) to get it to scale up correctly.
you should see the rectangle scale out to cover the whole
line as the animation plays.
With both Mask and Line layers Locked, you should see the
mask reveal the line over time.